The cell damage of the cytostatics Bleomycin and ICRF 159 were studied experimentally by the use of the following parameters: colony survival, colony-size-spectrometry and multiplicity of CHO-Fibroblasts. Series of increasing drug concentrations and simultaneous irradiation with 200 rds were measured. In the above mentioned tests an additive and no potentiating effect of damage by the cytostatics in combination with irradiation of 200 rds was found. Compared with Bleomycin, ICRF 159 showed a higher degree of damage to CHO-Fibroblasts.